During the process of welding a metal, it is common for the metal to become discoloured by the welding process. For example, when welding stainless steel, the chromium content of the metal is depleted, exposing iron and causing a localised discolouration of the stainless steel, known as “heat tint”. In other applications, oxides form during the welding process, leaving unattractive deposits on the surface of the metal, or otherwise discolouring the metal.
It is common practice that after a metal has been welded, the discoloured surfaces are cleaned to remove or reduce the heat tint. In the past, this has typically been performed by a variety of methods, such as using an abrasive, bead blasting or exposing the discoloured region to strong acids. Electropolishing machines that apply an electrolyte in combination with a pad to portions of metal have also been used for mile discolouration. More recently, cleaning of welded joints has been performed using an electrically charged brush having conductive filaments, in conjunction with an electrolyte, to clean a metal surface by applying a concurrent chemical reaction, heat and electric current. The simultaneous electrolytic and high temperature cleaning action has proven successful in efficient cleaning of welded metals, particularly in cleaning stainless steel.
One method used to clean welded metal surfaces is disclosed in International PCT patent publication no. 2005/089968 entitled “Conductive brush for cleaning metals” in the name of co-applicants Fornasari and Bunting. This document describes the use of a brush containing conductive filaments connected to an electrode and a source of cleaning solution. The conductive filaments of the brush are used to apply electrical current and cleaning solution to a discoloured surface of a metal, whilst also brushing the surface, to remove localised discolourations.
Whilst the charged brush disclosed by this prior published patent application would likely prove an effective method of cleaning a metal, the filaments of the brush are constantly degraded during the process, consuming filament material. Accordingly, brush operators must monitor filament length and replace a brush when required. This is inconvenient, time consuming and potentially excessively costly. Having to regularly replace brushes can also prove particularly problematic in relation to automated cleaning, where an operator is not necessarily present to replace a brush, potentially causing machine down-time. Furthermore, the conductive filaments of the disclosed brush are relatively flexible and have a tendency to splay out. This reduces the concentration of the electric current (‘current density’) and reduces the cleaning speed of the method.
International PCT application no. 2010/085849 entitled “Conductive brush for cleaning metals” in the name of Kezza Products Pty Ltd discloses an alternative conductive brush for cleaning welded stainless steel. The brush in this patent application has a body connected to a fixed length of conductive brush filaments and a sheath arranged around the filaments. The sheath has an aperture through which the filaments extend having a profile of a particular geometry to shape the filaments retained within the sheath. The sheath is also movable with respect to the body and filaments, the movement adjusting the portion of brush filaments that protrude from the aperture, adjusting the effective length and stiffness of the filaments.
The brush disclosed by this patent application offers some advantages over the prior art as the sheath allows the active length of brush filaments to be adjusted, which adjusts the brush stiffness. Also, as the sheath aperture has a particular cross-sectional profile, the filaments are retained within this profile. The adjustment of the sheath therefore enables a user to adjust the properties of the brush according to the cleaning task and in particular, allows a user to shorten the filaments to create a stiff brush having a particular shape for specific cleaning requirements, such as precise application of the bush, and increasing the current density applied by the brush.
Whilst these advantages are beneficial to a conductive brush for cleaning metals, the brush disclosed by this patent also has a number of drawbacks. For example, the position of the sheath with respect to the filaments or body is freely adjustable and is unable to be locked. During use, this requires a user to manually maintain the position of the sheath or else the sheath may move from the desired position, releasing the filaments from the desired, precise brush geometry. Accordingly, it would be advantageous to have a locking mechanism to hold the sheath in place.
Also, the disclosed invention relates to a brush having a single brush head only. This is very restrictive, as the cleaning area is limited to the size of a single brush only. It would be advantageous to have a conductive brush assembly which may have a single or multiple adjustable brush heads to increase the cleaning area or create more complex geometry brushes for particular cleaning tasks.
The disclosed invention is also limited with regard to the length of brush filaments it is able to provide, by the length of the sheath. If the conductive filaments are longer in length than the sheath, the sheath is no longer able to contain the filament geometry or regulate the stiffness of the filaments. Accordingly it would be useful to have an arrangement whereby the brush length could be much greater than the length of the sheath, allowing the replacement interval for brushes to be greatly extended, or avoided altogether.